There is a widespread need for the rapid and accurate detection of the presence of drugs in organisms, including humans. Some drugs may have an optimum window of concentration, within which they have maximum therapeutic effect with minimal side effects. Some drugs may have a threshold concentration above which their long term use can be harmful to the health of the patient. Still other drugs are illegal or are otherwise forbidden or restricted by regulating agencies. Measurement of the presence or amount of a drug in a subject can be accomplished by the analysis of bodily fluids.
Often, the drug of interest is present in a low concentration, making it difficult to obtain an accurate analysis. For example, drugs are often extensively metabolized in an organism, resulting in low concentrations of the drug in urine and plasma samples, and only trace amounts of drugs in saliva samples.
Drugs and/or their metabolites can be detected accurately through Gas Chromatography (GC) and High Pressure Liquid Chromatography (HPLC); however, these methods are expensive and time consuming. Thus, immunoassays for the analysis of drugs in urine and plasma are widely used. Immunoassays can rapidly detect the parent drug compound along with other structurally related drugs, including their metabolites. In general, immunoassays measure the binding between an analyte, such as a drug or drug metabolite, and an antibody for the analyte. These measurements may be done directly, by the detection of the analyte-antibody complex; or they may be done indirectly, by measuring the change in binding of the antibody and an analyte derivative, where the change is due to the presence of the analyte. Immunoassays typically involve the analysis of liquid samples. The liquid may be a free-flowing liquid in a container, or it may be impregnated within a porous or discontinuous solid phase.
Chromatographic immunoassays, which incorporate the use of a porous matrix material into conventional immunoassay techniques, are described, for example, in U.S. Pat. No. 5,770,458, which is incorporated by reference herein. In this format, a complexing reagent is bound to a region of a porous matrix material such as a fibrous or porous membrane. The complexing reagent is either the antibody to the analyte of interest or a derivative of the analyte that has been labeled to allow it to be detected. The liquid sample containing the analyte is loaded onto the matrix material in a region away from the bound complexing reagent and is allowed to migrate through the porous carrier to the region containing the bound complexing reagent. A second complexing reagent may be added to this fluid flow due to its presence within or adjacent to the matrix material or due to addition by the user. The second complexing reagent may also be an antibody to the analyte or a labeled analyte derivative. The measurement of the presence and/or concentration of the analyte can thus be based on detection of complexation between the analyte and two different antibodies (sandwich), the complexation between the analyte and one antibody (direct), or the change in expected complexation between an antibody and a labeled derivative of the analyte (competitive).
The detection of drugs by immunoassays, including chromatographic immunoassays, requires a drug standard. A solution having a known concentration of the drug standard in a buffer formulation is prepared and stored. This concentration and the measured response of the assay to the standard are used to calculate the amount of drug in the test sample. This calibration may be performed before, during, or after the analysis of the sample. For a chromatographic immunoassays, which are typically configured as single-use strips, the calibration may be performed on a representative sampling of the strips as part of the manufacturing process. The standard solution of a predetermined amount of a drug can also be used as Quality Control material.
Drugs which are not readily soluble in water, referred to as hydrophobic drugs or lipophilic drugs, are often difficult to measure by chromatographic immunoassay, since a constant amount of fully soluble drug standard may be difficult to maintain. This solubility behavior has an adverse effect on the determination of analytes in immunoassays and is particularly troublesome in immunochromatographic detection, where the standard solution comes into contact with surfaces such as absorbent pads and porous matrix material. The compound may not stay in a homogeneous state, and an accurate concentration of the standard cannot be consistently maintained. Consequently, the consistency and reproducibility of the drug determination is compromised.
It is thus desirable to provide standards for lipophilic drugs that are useful in immunoassays. It is desirable that these standards are water-soluble. It is also desirable that these standards have adequate mobility under chromatographic immunoassay conditions, and that they are stable in water, specifically in a physiological environment. Such standards ideally will interact specifically with the antibodies that are used in the assays.